Cycling of implantation step and remote plasma process step for nitride spacer etching applications.

Abstract

The etching of silicon nitride spacers is one of the most challenging steps of transistor fabrication. It requires anisotropy to preserve the sidewalls and a high etch selectivity over the underlying substrate to achieve a high surface quality. Recently, an interesting approach using a two step-process was proposed for the etching of silicon nitride spacers with high anisotropy and minimal induced damage [1]. The first step uses an H₂ implantation to selectively modify the horizontal SiN surfaces over the vertical ones, while the second step selectively removes the modified layer either via HF exposure or via a remote plasma (RP). This paper explores a new route to implement those two steps in a cycling process achieved in the same plasma reactor chamber. The reactor has the capability to produce both a capacitive plasma discharge (CCP) for the implantation step and a remote discharge for the removal step. This study demonstrates that the remote plasma process, whose etching mechanisms are driven by reactive neutrals, is highly sensitive to the material surface state and consequently an incubation time exists before the etching starts when exposed to neutrals. The modifications induced by the first implantation step shortens the incubation time offering a process window with infinite etch selectivity between horizontal implanted and vertical non-implanted surfaces. Based on this understanding a two-step cycling process was developed and applied successfully to the etching of Si₃N₄ spacer patterns for imager applications.